For communication systems, Time Division Duplexing (TDD) is a mode of communication operation where transmission and reception are separated in time. This separation typically means that certain parts of the modems can be simplified compared with Frequency Division Duplexing (FDD) systems, which transmit and receive simultaneously but at different frequencies. Such simplification is desired since it saves both costs and valuable circuit board space. In the analog front end, TDD systems can omit the costly hybrid, which separates the transmitted and received signals in FDD systems like VDSL2, see reference [1].
In the digital domain, OFDM-based TDD systems do not need separate FFT/IFFT blocks for reception and transmission but can reuse the same block since there is a simple relation between the Fast Fourier Transform (FFT) and the Inverse Fast Fourier Transform (IFFT).
Single-Ended Line Test (SELT) typically comprises measurement of echo (input reflection) and Quiet Line Noise (QLN).
The only SELT method that is compatible with the TDD feature of transmission and reception in separate timeslots is Time Domain Reflectometry (TDR). TDR systems typically transmit a short high-energy pulse since the pulse length determines the resolution and the pulse energy determines the reach.
A pulse is transmitted to the line (with receiver turned off) and a switch from transmission to reception is made shortly after the transmission of the TDR pulse. Many commercial TDR measurement instruments are also designed in this way.
FIG. 2 illustrates a known TDD device 10 operating according to the TDD principle applied to an Orthogonal Frequency Division Multiplexing (OFDM) based system. The device 10 comprises a transmitter 14 and a receiver 18. The transmitter 14 and the receiver 18 operate alternately as a switch 16 alters the connection to the transmission line 20. There is one common IFFT/FFT block 12 either operating in the IFFT mode during transmission or in FFT mode during reception. The IFFT/FTT covers the whole bandwidth for downstream and upstream transmission.
One problem when implementing TDR in a communication modem is the transmission of the narrow high-energy pulse which put high demands on the analog front end, especially the transmit amplifier.
Another problem is that it will have a dead zone where no echoes will be visible. This dead zone is determined by the length of the pulse and the time needed to switch from transmission to reception. Further, the switching time for a communication modem may be much longer than for a tailored TDR measurement instrument since the modem is typically designed based on the requirements for communication operation.